The detection of misfires in all combustion engines has become a major problem, owing to the most recent or pending legislation regarding environmental protection with which these engines, associated with catalytic converters, have to comply.
Now it is known that the occurrence of misfires may lead to deterioration of the catalytic converter, or even destruction thereof in the event of successive misfires on account of quantities, which may be excessive, of unburnt fuel mixtures resulting from these misfires reaching the catalytic converter. This deterioration or destruction of the catalytic converter leads to uncontrolled emanation of pollutants in the exhaust gases, and therefore to the vehicle no longer complying with legislation.
The overall problem upon which the invention is based is that of proposing a method and a device for detecting misfires, making it possible to alert the driver in the event of a risk of degrading the catalytic converter through misfiring and/or of excessively increasing a polluting emission.
From WO-A-94 16209, there is already known a method for detecting misfires in a controlled-ignition internal combustion engine, comprising the steps consisting in:
a) formulating, for each combustion/expansion phase in each cylinder of the engine, a signal representative of the value of the gas torque Cg produced in the corresponding cylinder by the corresponding combustion, PA1 b) measuring the rotational speed N of the engine and formulating a signal representative of this speed for at least the inlet phase which preceded the combustion/expansion phase in the corresponding cylinder, PA1 c) measuring a parameter representative of the unitary filling of the corresponding cylinder and formulating a signal representative of this unitary filling for at least the inlet phase which preceded the combustion/expansion phase during the same engine cycle in the corresponding cylinder, PA1 d) entering the signals representative of speed N and of unitary filling as addresses in a reference theoretical gas torque map, and deducing therefrom a signal representative of a reference torque Cref. PA1 e) assigning to the reference torque Cref a multiplicative weighting coefficient K associated with the corresponding cylinder and/or with whether the engine is running in stabilized or transient mode, and in deducing therefrom a signal representative of an expected theoretical torque Cpr, and PA1 f) comparing the signals of the measured gas torque Cg and of the expected theoretical torque Cpr and deeming there to be a misfire when the ratio of the gas torque Cg to the expected theoretical torque Cpr drops below a given ratio threshold. PA1 e) assigning to at least one of the signals representative of the value of the gas torque Cg and of the reference torque Cref a respective multiplicative correction factor KM associated with the corresponding cylinder 80 as to obtain signals which are proportional to the value of the gas torque Cg and to the reference torque Cref, and, in addition, assigning to at least one of the proportional signals, an additive correction factor KA when the engine is running in transient mode, and deducing therefrom a signal representative of an expected theoretical torque Cpr on the basis of the signal which is proportional to the reference torque-Cref, and a signal representative of a calculated gas torque Cgc on the basis of the signal which is proportional to the value of the gas torque Cg, PA1 f) calculating the ratio of the signal representative of the expected theoretical torque Cpr to the signal representative of the calculated gas torque Cgc, and deeming there to be a misfire when this ratio is higher than a given ratio threshold. PA1 in measuring the number of misfires or level of misfires detected during windows of respectively N1 and N2 engine revolutions, for example 200 and 1000 revolutions, PA1 in comparing each of these measurements with at least one threshold specific to each window, and PA1 in giving a warning signal corresponding to a critical state of the catalytic converter in the event of the threshold specific to the N1-revolutions window being exceeded, and/or a warning signal corresponding to a threshold of the emission of harmful components in the exhaust gases being exceeded in the case of the threshold specific to the N2-revolutions window being exceeded. PA1 at least one sensor for sensing the rotational speed of the engine, delivering a speed signal N, PA1 at least one sensor for sensing a parameter representative of the unitary filling of the cylinders of the engine, such as a sensor sensing the pressure at the air inlet manifold, an air inlet flow meter or a sensor for sensing the butterfly valve angle, delivering a signal representative of this parameter, PA1 means for calculating the gas torque Cg, on the basis of the engine speed signal N, PA1 memory means for storing maps and thresholds, calculation means and comparison means making it possible to calculate and/or to store values of the reference torque Cref, of the expected torque Cpr, of the calculated gas torque Cgc, of the multiplicative correction factor (KM) and additive correction factor (KA) and of the Cpr/Cgc ratio and to compare them with thresholds in order to determine the presence of misfires.
This known method consists additionally in: